1. Field of the Invention
The present invention relates to the prevention of atmospheric fog formation which is a common problem when vaporizing cryogenic fluids and Liquefied Natural Gas (LNG) using ambient air by using the cold energy of the cryogenic fluid to cool and dry the ambient air flowing to the vaporizer and using induced draft fans which reheat the cold air leaving the vaporizers with a side stream of warm ambient air. In the process, the LNG is preheated before entering the ambient air vaporizer.
2. Description of Related Art
Eyerman in U.S. Pat. No. 6,644,041 B1 shows a process for vaporizing liquefied gas (LNG) using heat from the ambient air by using an air evaporative heating tower to warm water which then transfers this heat to an intermediate fluid within a closed loop which in turn transfers its heat to the LNG which is vaporized. The heating tower is similar to the more common cooling tower art for waste heat dissipation. Such towers have the problem of recycling discharge air back into the air intake.
Iwasaki in U.S. Pat. No. 6,164,247 shows an intermediate fluid type vaporizer wherein the intermediate fluid temperature is maintained by passing sea water through the vaporizer-heat exchanger, the sea water thus supplies the heat required for the vaporization process. In many locations adequate temperature sea water is not available and environmental and corrosion issues are a concern.
Mandrin in U.S. Pat. No. 3,978,663 uses the cold energy from a vaporizing LNG stream for vaporizing the LNG which vaporized gas is fed to a turbine. The LNG is vaporized using ambient air as the heating means and subsequently the cooled air is fed to the turbine air intake, thus increasing thermodynamic efficiency. A similar process is described in Yamani U.S. Pat. No. 5,400,588
A sea water heat source is used in an LNG vaporizer described in U.S. Pat. No. 5,341,769 to Ueno. Ueno in FIG. 13 shows the relative temperature profiles of the sea water and the LNG in the counter-current flow stream in these falling (water) film type heat exchangers.
Solar energy has also been proposed as a heat source for LNG vaporizers in U.S. Pat. No. 4,331,129 by Hong. Hong discloses a solar pond which collects local solar radiation to maintain the water or other heat transfer fluid temperature to provide the required energy for vaporization.
Heating Towers using ambient air as a heat source to warm a circulating water loop in direct contact with the air are described by Mockrey in U.S. Pat. No. 7,137,623B2 and further in U.S. Pat. No. 7,431,270B2. Mockrey discusses the problem of cold dense air sinking to ground level where it can be drawn back into the cooling tower thereby reducing tower efficiency, such recycling being a common concern of ambient air heat exchangers.
Fog dissipation is widely discussed in prior art particularly concerned with naturally occurring fog banks and ground fog. For example Vielberth in U.S. Pat. No. 5,810,248 illustrates a water spray method to prevent or eliminate fog over an area of land such as an airport, roadway or large building complex. Likewise in U.S. Pat. No. 3,598,313, Plattner uses a cold air stream, which is more dense than the ground fog layer, the cold stream dispersed above the area that is to be cleared of fog, thereby clearing the fog from the area. Additionally, mixing between the top fog layer and the bottom cool-dry layer causes some of the visible fog (water) to be converted into invisible water vapor. Brown in U.S. Pat. No. 7,870,747 B1 discloses an ambient air vaporizer for liquefied gas. In the disclosure, the cold, dense, air from the vaporizer is fed via a duct into an air re-heater prior to discharge of the warmed air to the atmosphere. The purpose of the air re-heating is to obviate objectionable fog production and an (air) psychometric chart shown by Brown in FIG. 1 outlines the range of air temperatures, enthalpy range, water content of the air, etc. Such charts are in common use in the air conditioning art. Brown discloses that ambient air as the re-heat source is theoretically possible, but does not further pursue the method.
An ambient air vaporizer, fitted with a restrictive air flow apparatus to increase the effluent air temperature from the vaporizer to reduce potential fogging is disclosed in Brown U.S. Pat. No. 7,493,772 B1. Other than fog potential reduction, the disclosure is silent on the performance of the vaporizer as the flow of the natural draft stream of heating air is reduced. One skilled in the art would expect a lower vaporizer performance to be consistent with restricted an flow. In White U.S. Pat. No. 5,390,500 a typical ambient air vaporizer is disclosed. White discloses such concerns as ice formation, performance, and potential fog generation and discloses internal cryogenic flow passage elements for ice management and possible fog reduction.